Many limb prostheses operate in response to muscle contractions performed by the user. Some prostheses are purely mechanical systems. For example, a type of lower limb prosthesis operates in response to the motion of the residual limb. When the user's thigh moves forward, inertia opens the knee joint of the prosthesis, an artificial shin swings forward, and, when the entire structure locks, the user may pass his or her weight over the artificial leg. Other prostheses may incorporate electric sensors to measure muscle activity and use the measured signals to operate the prosthesis.
Such prostheses may provide only crude control to users that have control over some remaining limb musculature, and hence may not be useful for patients with spinal damage. For these patients, it may be desirable to measure precursor signals coded for limb movement in the patient's brain, and then decode these signals to determine the intended movement and/or target. The decoded signals could be used to operate a pointing device on a computer display screen or robotic prosthesis.